The ability to estimate the physical location of a mobile terminal in a wireless communication system finds broad utility across a range of applications, including emergency dispatch services, navigation systems, information or marketing services, and others. Many of these applications benefit from highly accurate location estimation, and indeed some even require it. In the United States, for example, standards adopted by the Federal Communications Commission (FCC) mandate that wireless service providers regularly estimate the location of a mobile terminal for emergency dispatch services within an accuracy of as little as 50 meters, depending on the type of location technology used by the provider.
Location fingerprinting offers perhaps one of the most promising approaches for reliably estimating location with such high accuracy, even in urban and indoor radio environments where other approaches fall short. Location fingerprinting is implemented in two phases: a training phase and a location estimation phase. During the training phase, the wireless service provider conducts an extensive survey of the service area to measure unique properties of the signal space at selected reference locations. At each reference location, for example, the provider may determine the set of radio access network nodes detectable from that location and measure the strength of signals received from each access node. Alternatively, the provider may measure the quality of those signals, the path loss, multi-path characteristics, or any other type of signal measurements with respect to each access node. The resulting set of signal measurements for a given reference location uniquely identifies that location as its radio fingerprint. The provider then stores the reference radio fingerprint of each reference location in a database together with the known coordinates of those locations.
During the location estimation phase, a mobile terminal performs the same sort of signal measurements to obtain a radio fingerprint of its unknown location. This radio fingerprint is then compared to the reference radio fingerprints to estimate the mobile terminal's location according to some pre-determined estimation algorithm. The single nearest-neighbor algorithm, for example, estimates the mobile terminal's location as being equal to that of the reference location having a reference radio fingerprint most similar to the radio fingerprint of the mobile terminal's location. Other estimation algorithms include the K-nearest neighbor algorithm, the weighted K nearest-neighbor algorithm, etc. By collecting reference radio fingerprints for reference locations chosen to be, e.g., every 3 to 5 meters, a wireless service provider can theoretically estimate a mobile terminal's location within an accuracy of just a few meters.
Some contexts complicate the above approach to location fingerprinting. For example, a wireless service provider often conducts the extensive site survey necessary for location fingerprinting when the wireless network is being deployed. As part of this survey, the provider obtains the reference radio fingerprints by performing signal measurements at one or more of the radio frequencies then available for use by the network and mobile terminal. New frequencies, however, can subsequently become available for use, e.g., if the provider acquires carriers at additional frequencies. This allows the provider to serve mobile terminals that support additional frequencies, carry out load balancing on different carriers with mobile terminals that support multiple frequencies, and otherwise increase capacity. Yet if the provider does not re-conduct the extensive site survey for these new frequencies, a time consuming and costly endeavor, a mobile terminal may obtain the radio fingerprint of its location by performing signal measurements at a radio frequency different from that used by the provider to obtain the reference radio fingerprints.
Many types of signal measurements used for location fingerprinting, such as signal strength and path loss, depend on the radio frequency at which they are performed. For example, two path loss measurements performed at the same location but at different radio frequencies may differ by as much as 10 dB. A difference in the radio frequency used for signal measurements, therefore, may cause significant error in the location estimated via location fingerprinting, on the order of hundreds or thousands of meters, threatening the ability of location fingerprinting to otherwise meet the accuracy demands of many wireless communication applications.